1. Field of the Invention
The present invention relates to tensioning devices for exerting constant and variable tensional forces, as for example between two relatively movable parts of a machine, and to mechanical systems including such tensioning devices.
2. Description of the Prior Art
There are many types of mechanical systems in which a constant tensional force must be maintained between two relatively movable members, as for example between stationary and movable parts of a machine or other mechanical system. Examples of mechanical systems in which this requirement exists range from precision measuring and testing instruments to agricultural machinery. Components of such instruments and machines often must be preloaded with a constant force device. This preloading force may have to be very nearly constant over a considerable uniaxial displacement distance. A particularly important need for constant force tensioning devices occurs in the aerospace industry, in which it is frequently necessary to maintain a constant level of tension in the control linkages or cables of an aircraft despite the effects of stretching, temperature-induced expansion and contraction, and material wear.
Conventional mechanical spring devices, such as coil springs and leaf springs, are generally characterized by a linear increase or decrease in the spring force as the spring displacement or deflection is varied. This relationship is usually expressed by the well-known equation F=Kx, where F is the restoring force exerted by the spring, x is the displacement distance, and K is the so-called "spring constant". It is apparent from this equation that conventional types of springs must be limited to small displacements, or deflections, if the restoring force is to remain reasonably constant. Consequently, conventional types of springs are poorly suited to systems in which constant tension requirements are imposed. See, for example, U.S. Pat. No. 3,141,352, which discloses tension maintaining devices containing doil springs, but points out that these devices will not provide a substantially constant tensional force unless the displacement distances are small and the springs are selected with care.
Special flat coil type springs, similar to clock springs, have been developed to satisfy a constant force requirement. Unfortunately, springs of this type are subject to extremely high local stresses when they are in use, leading to premature and catastrophic failure. In applications requiring rapid actuation or high cycle life, therefore, this type of spring is generally unsatisfactory. The possibility of early and catastrophic failure is particularly unsatisfactory in applications which require high reliability, such as in the aerospace industry.
A variety of other types of devices have been proposed to satisfy constant tension requirements. In U.S. Pat. No. 4,274,300, for example, a device consisting of a reciprocable arrangement of rotary cam surfaces and a fixed follower pin is provided for automatically taking up slack in a cable system. However, the device is adjustable only in increments, rather than continuously, and it requires a positive actuating movement of the cable system before the desired self-adjustment will occur. Another example of a tension-maintaining device for cable systems may be found in U.S. Pat. No. 2,515,274, which proposes a fluid-filled hydraulic unit including a spring-controlled piston and valve member and an expansible and contractable volumetric chamber vented to the atmosphere. When a sudden load is applied to the cable to which the device is attached, the piston valve closes and the device becomes a solid hydraulic unit by virtue of the incompressible fluid (e.g., oil) therein. Under no-load conditions, the piston valve is open and the spring operates to maintain cable tension by moving the piston in one direction or the other. The use of an ordinary coil spring as the primary tension-maintaining element, however, means that the restoring force will vary somewhat as the piston moves, rather than remaining substantially constant. The expansible volumetric chamber compensates for the displacement of the fluid medium within the unit (e.g., due to withdrawal of the piston rod or changes in temperature and barometric pressure), but does not compensate for the variable force characteristic of the coil spring. A further example of a tension-maintaining device for cable systems may be found in U.S. Pat. No. 2,395,261, in which the proposed device consists of a cylindrical tube or casing made of a metal having a high coefficient of thermal expansion (such as aluminum) surrounding a longitudinal rod made of a metal having a low coefficient of thermal expansion (such as Invar). The tube and rod are connected by an arrangement of levers which produce a mechanical advantage that is effective to maintain constant tension in a cable attached to the device. The disadvantage of this device is that it is responsive only to temperature changes and does not compensate for other factors affecting the cable tension, such as gradual stretching of the cable over time.